Fix bug when trying to get job result in binary format

[MartinPontius]

Overview

Currently, none of the existing process managers (TinyDB, MongoDB, PostgreSQL) is able to return binary process outputs on a GET request to /jobs/<job-id>/results (get_job_result method of the managers). This is especially problematic for asynchronous processes as there seems to be no way to actually get the process outputs. For synchronous process calls, binary data can already be returned on POST requests to /processes/<process-id>/execution (see).

This PR should solve the issue for all three aforementioned managers.

Related Issue / discussion

Additional information

Dependency policy (RFC2)

• [x] I have ensured that this PR meets RFC2 requirements

Updates to public demo

• [ ] I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • [ ] https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• [ ] I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• [x] I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines